The long term goals of this Program Project are to define the domains of cells specified to particular fates in a vertebrate embryo, and to learn how these domains are initially patterned, subsequently refined, and progressively subdivided such that individual cells can finally express highly restricted fates according to their positions. During the current project period it has become apparent that widely conserved intercellular signaling molecules are used at each of the developmental steps, and that the signaling interactions occur in hierarchical cascades. Understanding how these signals act and interact, and the logic of the signaling hierarchy, is critical for understanding developmental patterning. The goal for the coming project period is to obtain this understanding for particular cell fates and regions of the embryo. The goal will be achieved in three Component Projects. Project by Kimmel: Signals that pattern the skull forces; Project by Westerfield: Signals that pattern the anterior central nervous system; Project by Eisen: Interactions among signaling pathways during neuronal specification. Each project proposes a genetically based developmental analysis using zebrafish. Gene expression analysis, and mosaic analyses involving transplantation of cells between wild-type and mutant embryos, will reveal the signaling cells and cells that respond to the signals Loss- and gain-of function experiments will establish hierarchical interactions among signals as diverse as nodals, hedgehogs, retinoids, thyroid hormones, and neurotropic factors. Defects in all these signaling pathways, as well as others that will be examined, are implicated in particular human diseases; hence, results from this research will have direct bearing on human health. Five Core Facilities, including a unique Genetics/Zebrafish Facility, will provide support for these Component Projects.